Forming conductive bumps on input and output terminal pads of semiconductor devices for the purpose of flip chip mounting is becoming more common. With an ever increasing demand for semiconductor devices with small foot prints, flip chip mounting is expected to increase in popularity, leaving the market for wire bonded and leaded semiconductor devices diminished. One known method for forming conductive bumps in flip chip applications is a technique wherein metal is evaporated onto a device's input and output terminal pads, and subsequently reflowed to form the conductive bumps. The evaporation is performed selectively, such that metal is only evaporated on the terminal pads and not on other portions of the device. The selectivity is accomplished through the use of an evaporation mask. Typically, the mask is a metal mask formed of, for example, molybdenum. The metal mask is patterned using conventional lithography and etching techniques to form openings through the mask which correspond to the pattern of terminals on the device onto which metal is to be deposited. While evaporation through a metal mask has found success in the formation of conductive bumps on semiconductor devices, the process has a significant limitation pertaining to the dimensions of the holes formed in the mask. This limitation and a further explanation of the metal mask technology is explained below in reference to FIG. 1 and FIG. 2.
FIG. 1 is a perspective view of a metal mask 10 used for forming conductive bumps on a semiconductor device. As illustrated mask 10 includes a plurality of openings 12 which extend through the entire thickness of the mask. The pattern of openings 12 correspond to the pattern of terminals on the semiconductor device onto which metal will be deposited through the mask. Mask 10 is typically patterned with openings to permit bumping of an entire semiconductor wafer having a plurality of die; however, for ease of illustration the individual die bump patterns are not delineated in FIG. 1. An exploded cross-sectional view of mask 10 taken along the line 2--2 is shown in FIG. 2. Openings 12 extend through the entire mask thickness, illustrated as "t". As illustrated in FIG. 2, each opening has an hourglass shape rather than being cylindrical with vertical sidewalls. The hourglass shape serves an important purpose of controlling the height and volume of metal used to form the conductive bumps. Having a uniform height and volume of conductive material on the terminals of the semiconductor device is important to achieve a reliable connection between the semiconductor device and a next level substrate. A further explanation of how the hourglass shape achieves tight height volume control is provided below.
The dimensional limitation of conventional metal mask technology is due the manner in which hourglass shaped openings are formed. To achieve the hourglass shape, metal mask 10 is simultaneously etched from both a top surface 13 and from a bottom surface 14 of the mask. Prior to etching, top surface 13 of metal mask 10 is patterned with a resist mask using conventional lithography techniques. The pattern of-the resist mask defines (either positively or negatively) portions of top surface 13 which are to be etched. Bottom surface 14 is likewise patterned with a resist mask; however, the pattern of the resist mask is a mirror image of that used on the top surface 13. The resist masks are mirror images of one another so that upon etching simultaneously from both sides of the mask, the portions etched from the top will meet and align with the portions etched from the bottom, thereby forming openings 12 which extend completely through mask 10. The etch used to define openings 10 is an isotropic etch which produces tapered sidewalls. The direction of the taper is inward from the direction of the etch. Since the etch is isotropic and performed from both the top and bottom surfaces, the openings 12 take on an hourglass shape as illustrated in FIG. 2.
With the foregoing etch and lithography techniques, metal masks such as mask 10 can be fabricated to form holes having a diameter of 125.mu. (5 mil) with a pitch of 250.mu. (10 mil). The hole diameter is typically measured as being the minimum opening diameter. For instance, in reference to FIG. 2, the diameter of openings 12 at either the top or bottom surface would be represented in FIG. 2 by "d". Pitch refers to the spacing between the center of adjacent openings and is represented in FIG. 2 as "p". While masks having 100.mu. (4 mil) opening diameters can be fabricated, the yield of producing such masks is significantly reduced. The reduction in yield results from processing limitations imposed by the diameter of the openings in relation to the thickness of the mask. At present, the aspect ratio between the opening diameter and the mask thickness (d/t) should be kept to a number greater than one (d/t&gt;1) in order to achieve acceptable yield levels. As a result, the lower limit on opening diameter is restricted by the thickness of the metal mask.
With the demand for smaller and more dense devices, there is also a need for smaller and more dense conductive bumps formed on these devices. However with the bumping technique described above, the lower dimensions of the conductive bumps formed through this process are restricted by the thickness of the metal mask. An obvious solution to this problem is to simply reduce the mask thickness, thereby allowing the diameter of openings to likewise be reduced. However, this solution is not feasible because the thickness of the mask is also used to control the height and volume of metal deposited through the mask openings (as further explained below). By reducing the mask thickness, the height and volume of metal deposited to form the bumps is likewise reduced, adversely affecting the ability to reliably connect a bumped device to a next level substrate. Accordingly, an improved process for forming smaller conductive bumps to increase terminal density is desired.